1. Field of the Invention
This invention relates to printing presses and more particularly to systems for applying dampening fluid to a rotating cylinder of a printing press.
2. Description of the Prior Art
In the offset printing process, a small amount of dampening solution, i.e., water with certain additives, is supplied to the offset plate, which then comes in contact with the inking rollers, the ink adhering to the image on the plate and the dampening solution adhering to the other portions of the plate. The quantity of dampening solution applied must be varied particularly in relation to the press speed. Control of the application of the dampening fluid is particularly important in four-color process, where variations will effect color. If too little fluid is applied, printing will occur in areas where none is desired.
Various systems for dampening the plate cylinder of an offset printing apparatus are in use today. One such system is described in Switall U.S. Pat. No. 4,064,801. This dampening system has a plurality of nozzles to spray dampening fluid on the plate cylinder. To supply dampening fluid through the nozzles, the system includes a tank and an air pump to pressurize the tank. In pressurizing the tank, the dampening fluid is caused to flow therefrom through a pressure compensator which controls the pressure of the dampening fluid according to the speed of the press. Speed of the press is determined by a tachometer which, through appropriate means, controls the adjustment of the pressure compensator.
One of the drawbacks of prior dampening systems is that continuous operation of the printing press requires a constant supply of dampening fluid at the correct pressure as determined by, for example, the speed of the press. Providing a very large tank puts demands upon the space requirements for the dampening system. Furthermore, pressurizing the large tank may present problems either from a delay standpoint where a small air pump is used to pressurize the tank to the proper pressure or from a cost standpoint when a larger air pump is employed.